Relative states, quantum axes, and quantum references
E. Bagán, S. Iblisdir, R. Muñoz-Tapia · Physical Review A · 2006
We address the problem of measuring the relative angle between two ``quantum axes'' made out of ${N}_{1}$ and ${N}_{2}$ spins. Closed forms of our fidelitylike figure of merit are obtained for an arbitrary number of parallel spins. The asymptotic regimes of large ${N}_{1}$ and/or ${N}_{2}$ are discussed in detail. The extension of the concept ``quantum axis'' to more general situations is addressed. We give optimal strategies when the first quantum axis is made out of parallel spins whereas the second is a general state made out of two spins.